A machine of this type normally includes an electrically driven motor and a hydraulic unit with an impeller connected to the motor via a rotary driving shaft. In order to prevent the medium within the hydraulic unit from flowing along the shaft and penetrating the electric motor, thereby causing damage, one or several seals are arranged between the motor and the hydraulic unit. A common type of seal is the so-called mechanical face seal, which comprises one seal ring rotating with the shaft and one stationary seal ring mounted in the surrounding housing. The two rings are pressed together by spring force thus preventing medium from penetrating between them.
If the medium within the hydraulic unit contains pollutants, a special problem occurs. As the pressure within the hydraulic unit is higher, pollutants may penetrate between the seal surfaces and cause damage, meaning that the seal is worsened or fails totally.
In order to solve this problem it is common to arrange two mechanical seals parted by a room filled with a barrier liquid such as oil, which lubricates and cools the surfaces. By this, the seal adjacent the electric motor will always operate with a clean medium and thus, the risks for damages will decrease drastically. If the seal adjacent the hydraulic unit should be damaged, medium from said unit may enter the barrier liquid room, but by controlling said liquid at regular intervals, the seal could be repaired or replaced before any serious damage has accured. An example of such a design is shown in the Swedish patent No 381 318.
If it has been noted that the barrier liquid has been too diluted by the medium in the hydraulic unit, the seal adjacent said unit must be replaced. If the dilution has been considerable, there is also a risk that the other seal has been damaged and therefore, it might be preferable to replace both seals at the same time.
In order to make such a replacement easier to obtain, it has been suggested to build them together into a unit which makes service easier and increases the reliability. Examples of such designs are shown in the Swedish patents 200 144 and 466 925. In order to obtain good circulation of the barrier liquid within the seal unit, it has been suggested to arrange a pump within the latter. Especially in a case where it has been chosen to use a closed cooling system for the electric motor using the barrier liquid as cooling medium, a pump is necessary if a sufficient flow should be obtained. Known designs such as those shown in Swedish patent 327 904 have, however certain disadvantages concerning space demand and efficiency.
This invention concerns a device which in an effective and secure way obtains the necessary circulation even at a low rotational speed and which has a very limited space demand.